Originally Posted by
John C. Ratliff
When the front wheel of a bicycle is suddenly stopped cold by an object (a stick in the spokes in this case), the whole bicycle rotates around that axis. The horizontal velocity of the cyclist in almost instantly translated to a rotational velocity, and the direction of rotation is directly into the ground.
I forgot to comment on this earlier but better late than never. I'm sure Brian Ratliff can agree with me here.
Without a force (in addition to gravity) pulling the cyclist towards the ground (or more precisely the pivot point of the front wheel), there is no way that a cyclist's horizontal velocity can be converted into a vertical velocity. There is the small force of the weight of the bike that, assuming the cyclist's hands stay wrapped around the handlebars and the bike stays on the ground, could slightly contribute to some extra vertical velocity but that's nothing compared to the force needed for a full transfer. If a cyclist's COG was positioned higher by going over the bars, that would also contribute to a higher vertical velocity by the time they hit the ground but that would require a handstand on the handlebars. I know my puny arms wouldn't let that happen
If you need help visualizing this, imagine a gymnast swinging around a horizontal bar. What would happen if they were to let go of the bar when their body was vertical above the bar? Would they still continue in a circle?